1. Field of the Invention
This invention relates to current amplifiers, and more particularly, to a current amplifier providing an output current corresponding to the difference between input voltage signals supplied across a pair of differential input terminals.
2. Description of the Prior Art
Certain circuits, such as variable impedance circuits, voltage amplifiers, voltage-to-current converters having electrically controllable gains, and instrument amplifiers which remove in-phase components from a balanced signal and convert the balanced signal into an unbalanced signal, include current amplifiers with multiplier circuits and voltage to current converters having differential inputs and differential outputs for amplifying currents supplied thereto. One such prior art current amplifier is described, for example, in SC-8, No. 4, IEEE JOURNAL OF SOLID STATE CIRCUITS, pp. 275-282 (August 1973). The current amplifier described in the IEEE JOURNAL is subject to fluctuations in the conversion factor, i.e., the ratio of the output current to the input voltage, and also to nonlinear components in the output signal. Accordingly, severe limitations are placed on the operating range and temperature of the disclosed amplifier.
The current amplifier disclosed in the IEEE JOURNAL is, however, well suited to an integrated circuit configuration because of improved thermal coupling and compatibility among the circuit elements. However, the transconductance of the circuit is not constant because the absolute value thereof depends upon the absolute value of the diffused resistance in the integrated circuit. The value of the diffused resistance, i.e., the resistor obtained by a diffusion process performed on the integrated circuit, fluctuates about plus or minus 20 percent. The diffused resistance also exhibits a temperature dependency on the order of about 1500 ppm/deg., so that the transconductance of the current amplifier in an integrated circuit configuration is subject to similiar fluctuations in its absolute value and exhibits a similar dependence on temperature.
Variations in the absolute value of the transconductance and in temperature do not pose problems for a current amplifier comprising a portion of a larger circuit contained completely within an integrated circuit. However, a very stable transconductance absolute value is important for a voltage amplifier in which the current output from the integrated circuit is converted into voltage by coil, capacitor or resistor elements which are disposed outside the circuit, or for a circuit in which the current amplifier is used as a variable impedance circuit for effectuating programmable frequency or filter characteristics.
Since the absolute value of the diffused resistance in an integrated circuit fluctuates about plus or minus 20 percent, and the transconductance of a current amplifier comprising an integrated circuit is primarily affected by the diffused resistance, a current amplifier constructed as a current output type integrated circuit is not completely satisfactory because its operation may be undesirably affected by fluctuations in the value of the diffused resistance.